These FMS flight management systems are well known. They make it possible to formulate the flight plan of an aircraft on each mission, by taking account of parameters specific to the aeroplane and to the flight conditions such as the payload, the weight of the aeroplane, the quantity of fuel on board, the temperature, the wind etc., and of the time constraints imposed by the ATC air traffic control bodies: departure and/or arrival time slot required.
Guidance to the flight plan is performed by considering target speeds for each phase of the flight: climb, cruising and descent. These speeds depend on the objective fixed by the company and achieve a compromise between flight time and fuel consumption. According to its objectives the company favours one or the other of these two parameters. Air operations involve a parameter corresponding to a cost index CI, defined as the ratio of the cost of the flight time to the fuel cost: CI=flight time Cost/fuel Cost.
This cost index is in fact a criterion for optimizing between the time costs CT (“Cost of Time”), in $/minute for example, and the fuel costs CF (“Cost of Fuel”), in $/kg for example. The Cost Index is defined by CI=CT/CF, with customary values lying between 0 and 999 (in kg/minute with the units indicated above). The value of this cost index for an aircraft and a given mission is determined according to criteria specific to each operator, and constrains notably the rules for determining the altitudes and speeds of the flight plan (vertical profile of the flight plan). Typically, a cost index CI equal to zero corresponds to a situation in which the cost of time CT is considered to be negligible with respect to the cost of fuel CF: flight planning will consist in seeking fairly low speeds for the least possible consumption, and the flight duration will be longer. For an operator, this typically corresponds to long-haul type flights. A cost index CI equal to 999 corresponds to an opposite situation, in which the cost of fuel CF is considered to be negligible with respect to the cost of time CT: flight planning will consist in seeking the shortest flight duration, even if the fuel consumption has to be high. For an operator, this typically corresponds to flights of shuttle type, so as to allow a maximum number of turn-arounds, or else to ensure an earlier arrival time in case of delay or precise landing slot.
The cost index CI therefore has a direct impact on the speed used for the climb, cruising and descent flight phases and consequently on the climb and descent flight profile as well as the duration of cruising through the positioning of the end-of-climb and start-of-descent points. However, this impact on the associated speed setpoint is not linear, for example a cost index CI of zero could lead to a speed of Mach 0.76, a cost index CI of 100 to Mach 0.82 and a CI of 900 to Mach 0.84. The relationship between the cost index CI and the speed is not easy for the pilot to grasp.
A problem arises in particular when the pilot is obliged to change speed because of an order from air traffic control, for example. The pilot can use the cost index CI, changing the latter to modify the speed of the aeroplane. This modification of the target speed is taken into account immediately by the aircraft with an effect on the engine regime. This represents a considerable drawback, notably at high altitudes, when the aeroplane flies under a regime close to the maximum engine regime. When the drop in speed is too considerable, a considerable lag is then necessary in order for the aeroplane to regain speed.
Moreover, the pilot of the aircraft is completely ignorant of the impact on the flight time, the difference in consumption and the Mach/speed discrepancy. The adjustment of the cost index CI and of the resulting parameters is therefore not intuitive.
An alternative consists in working to a secondary flight plan. A secondary flight plan is a copy of the active flight plan allowing the cockpit crew to modify the flight plan manually to take account of new characteristics or parameters, while ensuring the permanence of the active flight plan, that is to say without disturbing the systems which take it as reference. Once the modifications have been terminated on the secondary flight plan, the latter can become the active flight plan and vice-versa by order of the cockpit crew. The use of a secondary flight plan to fine-tune a new cost index is hardly practical. This makes it essential to use the same flight plan as the active flight plan not allowing calculation and direct display of the discrepancies in remaining flight time, and speed and consumption discrepancies. Moreover, this manipulation monopolizes a secondary flight plan, which is no longer available for other activities.
In the current design of certain flight management systems, it is possible to use a function known as Constant Mach segment allowing the pilot to specify a speed setpoint over a portion of the cruising phase, at best until the end of cruising. This function is not a regime optimization function but a way of applying an air traffic control speed constraint. It does not generate the calculation of a cost index CI and does not directly present any discrepancy in predictions.
The invention is notably aimed at alleviating the problems cited above by proposing a method and a system aimed at improving the understanding of the choice of the cost index CI, through an interface that does not disturb the current flight regime of the aircraft. The method and the system according to the invention calculate the modifications on the main predictions of flight cost-effectiveness: flight time and fuel consumption.